In typical color printers, supply drawers of one or more types of substrates are provided within a housing of the printer. Substrates pulled from one of these drawers are moved by a media transport through the printer in a process direction. Data of images to be produced on the substrates are transformed into color separations having pixels and these transformed data are transferred by a printer controller into a firing signal generator for selective operation of the ejectors in a printhead. The printheads are usually organized into staggered arrays and each staggered array extends across the media transport path in a cross-process direction that is perpendicular to the process direction in the plane of the substrates being carried by the media transport. Each staggered array also corresponds to a particular color of ink. For example, a printer having four staggered arrays includes one or more printheads having an array of ejectors in each printhead that eject ink directly onto the substrates as the substrates pass the printheads. Each staggered array of printheads corresponds to a single color of ink, one for each of the colors typically used in color printing, namely, cyan, magenta, yellow, and black (CMYK).
The signals operating the ejectors must be appropriately timed with the passing substrates. As the media transport carries the substrates toward the printheads, the media substrates pass a substrate detector that generates a signal in response to each leading edge of a substrate being detected. Additionally, the printer controller receives data from encoders mounted proximately to rollers positioned along a portion of the media transport to calculate the linear velocity and position of the substrates as the substrates move past the printheads. The position calculated by the controller is used to operate signal generators known as dot clock generators. These dot clock generators produce signals that enable a firing signal generator to operate a row of ejectors with reference to the transformed image data delivered to the printheads as an appropriate area in the cross-process direction passes the row of ejectors.
In previously known color printers, the dot clock generators for the different staggered arrays, sometimes called stations, count clock signals to synchronize the generation of the dot clock signals with the arrival of the substrates opposite the stations based on the velocity calculation. A problem arises when the media transport slips or hesitates as the dot clock generators continue to count clock signals even though the substrates have briefly slowed. Thus, the dot clock generators begin to generate dot clock signals that operate the ejectors in a row slightly before the appropriate position on the substrate is positioned opposite the ejectors appropriately. Thus, pixels of one color may not land adjacent the pixels of another color to produce a proper hue of a secondary or tertiary color. Generating dot clock signals with compensation for transport slips would be beneficial.